A backplane system includes a set of connectors that are disposed on a common plane—a backplane—and removable cards that plug into the connectors, such as memory cards, storage cards, processor cards, interface cards, interconnections with other backplanes, and the like (all generally referred to as cards herein). Typically, the connector provides to the inserted card everything that the card needs to function, such as power and access to system buses and other connections. In this manner, the cards can be easily inserted and removed as needed, such as to replace damaged cards, add redundancy, add capacity, or add new features to the system.
The backplane portion can be a chassis, which is often a relatively passive device that provides services to the cards, or can also be a more active device such as a motherboard. The term chassis is used herein as shorthand to include all such backplane devices, whether active or passive.
It is sometimes important for a given card to know the address of the slot on the chassis into which it is inserted, which can be accomplished by means of a slot coding mechanism. A well-known way to identify a slot is to measure the voltage that results from a voltage divider with one resistor on the card and the other resistor assigned to one of the pins in the slot on the chassis. Each slot has a different resistor value, so the card can determine the slot address via a simple voltage measurement.
However, sometimes it is also important for a given card to know the type, address, or some other identifier of the chassis into which it is inserted, so that it can properly function with the chassis. Currently, a second code mechanism is required to identify the chassis. The second code mechanism uses a second resistor on a second pin of the slot. Other methods use active components, like an EEPROM, to store the identifying information on the chassis. Both of these methods require additional interfaces, pins, and hardware.
What is needed, therefore, is a system that reduces issues such as those described above, at least in part.